1. Field of the Invention
The present invention relates to photo detectors for charge coupled device sensors. In particular, the invention relates to an active gate photo diode.
2. Description of Related Art
In FIG. 1, a known photo gate includes substrate 2, channel 4 formed in substrate 2, insulator 6 formed over channel 4, gate electrode 8 formed over insulator 6 and connection 10 through which voltage V1 is applied to gate electrode 8. Typically, substrate 2 is lightly doped silicon of a p type conductivity. Substrate 2 may be an epitaxy layer formed on other silicon or on an insulator. Substrate 2 may be a well of a p type conductivity formed in other silicon. Typically, channel 4 is of n type conductivity and typically formed in the substrate by ion implant or dopant diffusion. Typically, insulator 6 is a combined film of silicon dioxide and silicon nitride, but may be formed of other insulators. Gate electrode 8 is usually formed of doped poly-crystalline silicon (i.e., poly). It would make no sense to use an aluminum layer as gate electrode 8 since aluminum would block light radiation, and the photo gate would not function. Quite often in an array of photo gates, gate electrodes 8, for an entire row of photo gates, are connected together with one elongate film of poly so as to provide the needed connection 10.
In CCD sensors, two of such gates are combined to make a two phase photo detector element, three of such gates are combined to make a three phase photo detector element, or four of such gates are combined to make a four phase photo detector element. Such detectors are covered by the doped poly-crystalline silicon that constitutes the gate electrodes.
Most existing CCD sensors have low sensitivity to blue and UV because their overlying poly-crystalline silicon electrodes are mostly opaque at these wavelengths. There are four common techniques used to overcome this limitation. Interline transfer (ILT) CCD sensors use photo diodes as the photo detector element. The ILT CCD couples the photo diode, either directly or indirectly, through a transfer gate to a vertical CCD shift register. Indirect coupling stores charge in an intermediate storage well before transferring the charge to a vertical CCD. Open pinned-phase (OPP) CCD sensors use pinned photo diodes in the ILT structure. Windowed poly-crystalline silicon CCD sensors, also called reticulated photo gate sensors, use poly gates as depicted in FIG. 1; however, a portion of the poly is etched away to open a window so that photons can reach the underlying silicon. The windowed or reticulated portion of the photo gate functions as a photo diode. Some CCD sensors have had their backsides thinned, and the light is exposed through the back side.
While the ILT, OPP and windowed poly-crystalline silicon CCD sensors have improved UV detection sensitivity with respect to a poly photo gate CCD sensor, their fill factors are reduced, and their UV performance still needs to be improved. While the back side thinned CCD sensors may have good UV sensitivity, the manufacturing process is expensive and the modulation transfer function (MTF) is reduced.